Without getting to biblically inclined it was, as zmjezhd pointed out, technically a Levirate marriage which was legal. It was indeed the spilling of the seed for his own means that was the crime.

Correct. A Levirate marriage maintained the line of the brother and his share in the national "salvation" - ie his right to a bit of the Promised Land. The children born would be counted as those of the dead brother not the living one, and that's why Onan "spilled his seed" - because those children would've competed with his own for the family resources.

In the 19th century some said "If God wanted that mankind would fly, He would had given us wings". There is also a penalty for those who induce others to sin, "better had a mill stone put around their neck and be thrown to sea"

The Good Book states that penalty was for "spilling semen into ground", am I right?

Not exactly, no. That is just a description of what Onan did so as not to impregnate his dead brother's wife. The penalty in Onan's case was for practising what was in effect contraception by premature withdrawal in order to deny his brother a continuing line of children, probably because they would have competed with his own children by his wife. It is not a law saying that if you spill your seed on the ground God will punish you, like for example the law about not coveting you neighbour's wife or stealing. It is a story about something that happened. The only law involved in this story was the law of Levirate marriage - that was the law he was breaking.

Hello again!: while in school we were told a story of student that died suddendly. When the other scholars were praying for him, he appeared from death and said "do not pray for me, I'm in hell, as I commited a sin the night I died". Jesuit teachers insisted that auto chiro erasty was to be blamed for the awful destiny...

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